Abstract: A biaxially oriented polypropylene film for a capacitor, in which ?dm and ?dt are each 0.60-1.70 and the value of the ratio (?dm/?dt) of ?dm and ?dt is 0.75 to less than 1.15, where ?dm is the kinetic friction coefficient in a longitudinal direction and ?dt is the kinetic friction coefficient in a width direction has the film thickness measured by a micrometer method of 0.5-3 ?m. The biaxially oriented polypropylene film for a capacitor has excellent voltage withstanding ability and is demonstrated in a capacitor application, and stable productivity and workability are ensured.

Abstract: A process produces polyester film which is effectively reduced in the disadvantageous lengthwise and widthwise heat shrinkages inherent in biaxially oriented film and which is excellent in flatness and mechanical characteristics. More specifically, process produces biaxially oriented polyester film by subjecting a biaxially oriented polyester film which has been drawn in the lengthwise and widthwise directions and heat-treated to widthwise relaxation and lengthwise relaxation by a method of shortening the clip intervals, wherein the biaxially oriented polyester film is held under stretching between the widthwise relaxation step and the lengthwise relaxation step.